1. Field of the Invention
This invention relates to elongated metallic members such as tubing, wire, and the like, and more particularly to the art of curing a thermosetting plastic powder coating which has been applied to the surface thereof in order to prevent corrosion.
2. Description of the Prior Art
In many instances severe corrosion of wire and tubing as a result of specialized uses presents problems, and it is, therefore, desirable to have a coated final product. From an economical standpoint, utilizing an organic polymer as a coating for tubing and/or wire, such as a thermosetting plastic powder coating, is considerably cheaper than galvanizing. However, unlike when coating material with a thermoplastic powder, where it is simply necessary to reach a given temperature to fuse the coating and no care is required to hold the melting temperature for any required length of time, thermosetting plastic coatings, which are of low molecular weight, but which on application of heat, form higher molecular weight, cross-linked chemical structures, require time at temperature to allow for polymerization or "curing" to take place. As a practical matter, extensive modifications to existing equipment are necessitated in order to implement furnaces to cure thermosetting resins while maintaining reasonable line speeds in the neighborhood of 30 feet per minute. Accordingly, the prior art has endeavored to develop a more satisfactory way to cure thermosetting plastic powder coatings and more particularly elongated metallic members having such coatings.
U.S. Pat. No. 3,560,239, in the name of W. K. Facer, et al, is exemplary of prior art curing methods. Briefly, Facer, et al teaches the use of an induction coil for heating the enlongated substrate after application of a liquid primer. This serves to dry the primer as well as provide a heated substrate as it is passed through a fluid bed of resin powder. The sensible heat in the substrate supplies the energy for melting and holding the powder particles to the substrate to form a coating. A second induction coil following the fluidized bed is used only to fuse the particles on small diameter wire where the sensible heat or "heat sink" from the heating by the first induction coil is insufficient to completely fuse or flow the particles of coating together. However, it is not possible to coat and cure a coating of thermosetting plastic particles by following the teachings of the Facer, et al patent in which the substrate is heated by induction and passed through a fluidized bed of thermosetting resin. This is so because there is not sufficient latent heat in the wire to cause the resin to polymerize because all of the available heat in the substrate is used to melt the powdered resin. The residual heat in the melted thermoset resin is insufficient to effect a cure. Increasing heat input to the wire prior to powder application will result in pick up of a heavier deposit of melted resin which remains uncured. Large quantities of energy are required in the form of heat to convert matter from one state to another. The melting of a solid in the form of a powder uses all the energy (heat) that is stored in the wire. Increasing heat content only allows more powder to be melted. As long as a phase change is occurring, solid to liquid, little energy in the form of heat is available to cure the liquified thermosetting resin.
The prior art of induction heating emphasizes efficiency which, in turn, relates to a coil design to maximize the heating rate. An induction coil so designed is unsatisfactory for effecting a cure of a coating of thermosetting plastic due to an inability to control heat input. Degradation of the coating by over curing or loss of adherence by under curing result.